The characteristics of thermal pinch effect of arc in welding process are studied to clarify its mechanism. A simplified model based on energy balance between ohmic heating and conduction loss in an idealized cylindrical arc is useful for discussing the behavior of thermal pinch effect of welding arc. Furthermore, two-dimensional numerical simulations of gas tungsten arc in two imaginary shielding gases, namely, an argon except for the specific heat and an argon except for the thermal conductivity are presented for clarifying the mechanism of the thermal pinch effect. The numerical simulations show the results that the greater specific heat of shielding gas leads to the greater arc constriction but the greater thermal conductivity leads to the arc expansion. It is concluded that the thermal pinch effect is an arc constriction in a consequence of the greater specific heat of shielding gas which is generally given in molecular gas, for example, CO₂.